1. Field of the Invention
The present invention relates to a vehicle motion control system for controlling vehicle motion, and more particularly to the control system which performs a steering control by braking to restrain an excessive oversteer and excessive understeer which will occur during, for example, cornering, by applying a braking force to each wheel of the vehicle irrespective of depression of a brake pedal, and which performs an anti-skid control to prevent a wheel from being locked during braking, by controlling the braking force applied to the wheel.
2. Description of the Related Arts
Recently, a vehicle is provided with a braking force control system for controlling the braking force applied to the vehicle to perform an anti-skid control, a traction control, a front-rear braking force distribution control, and etc. In the U.S. Pat. No. 4,898,431, for example, an apparatus for controlling vehicle motion through the use of a brake controlling system which compensates for the influence of lateral forces on the vehicle. The apparatus is constituted so as to control the braking force applied to the vehicle by the braking force control system in response to a comparison of a desired yaw rate with an actual yaw rate, thereby to improve the vehicle stability during the course of vehicle motion such as cornering. Consequently, a braking force is applied to each wheel irrespective of depression of a brake pedal, so that a so-called steering control by braking is performed to provide an oversteer restraining control and understeer restraining control.
With respect to a braking force control apparatus for applying a braking force to each wheel of a vehicle in response to a braking condition of the vehicle to correct the vehicle motion, it is proposed in a Japanese Patent Laid-open Publication No. 7-117654 to correct an abnormal vehicle motion rapidly and properly by introducing a feedforward control in addition to a feedback control. In the apparatus, a feedforward control signal is output in response to a desired slip rate, and coupled with a feedback control signal which is output separately from the feedforward control signal, to produce an output signal to a hydraulic pressure control apparatus.
In the recent vehicle, also provided are an anti-skid control mode wherein a hydraulic braking pressure supplied to a wheel brake cylinder is controlled in response to a rotational speed of a wheel so as to control the braking force applied to the wheel, and the like modes, so that the anti-skid control mode and the like are combined with the mode of steering control by braking to provide the effective vehicle motion control system as a whole. In this case, when a pressure decreasing operation for the anti-skid control is performed, a pressure decrease compensating control for modifying an amount of pressure to be decreased is provided so as to prevent an excessive amount of decreased pressure which will result in an excessive decrease of the wheel acceleration and cornering force. Whereas, a pressure increase compensating control is performed after the pressure decreasing operation is terminated, so as to increase the braking pressure in response to a total amount of the decreased pressure, so that an amount of pressure to be increased is modified in response to a total time of the pressure decreasing operation performed until the previous cycle, wheel acceleration, coefficient of friction of a road or the like, for example.
However, the braking force applied to the individual wheel is to be controlled in accordance with one of the control modes. Therefore, in the case where the anti-skid control mode is selected when a certain wheel is under control in accordance with the steering control by braking mode, for example, only the pressure decreasing operation shall be performed for the steering control, because the braking force can not be increased during the anti-skid control mode, whereas excessive pressure decreasing operation will result in the decrease of the braking force. This results from the difference between the anti-skid control mode which is provided for decreasing the braking force applied to the wheel which is about to be locked, and the steering control by braking mode which is provided for applying the braking force to the wheel so as to control its steering angle. In this case, therefore, it is necessary to select the anti-skid control mode in advance, but it is not desirable to terminate the steering control operation.
FIG. 16 illustrates a .mu.-S curve which indicates a relationship between a coefficient of friction .mu. of a road or cornering force CF and a slip rate S. And, FIG. 17 illustrates a condition under an anti-skid control with respect to a certain wheel, wherein points (a)-(d) correspond to the points (a)-(d) indicated on the .mu.-S curve in FIG. 16. In FIG. 17, Vso** indicates an estimated vehicle speed, Vw** indicates a wheel speed, Wc** indicates a wheel cylinder pressure. In FIGS. 16, 17, point (c) indicates a peak value of the coefficient of friction (.mu.-peak) at which the anti-skid control starts, point (d) indicates an end of rapid pressure decreasing operation, point (a) indicates a transitional point from a hold (or, pressure decreasing operation) to a pressure increasing operation, and point (b) indicates an end of a pressure increase compensating control. As shown in FIG. 17, a zone from the point (c) to the point (a) corresponds to the zone for performing the pressure decrease compensating control, while a zone from the point (a) to the point (b) corresponds to the zone for performing the pressure increase compensating control.
In view of the fact that the cornering force CF is rapidly decreased in the transition from the point (a) to the point (d) as apparent from FIG. 16, the steering control by braking can be performed. In this case, in order to decrease the cornering force CF rapidly, the transition from the point (a) to the point (c) or (d) is to be made immediately. It is, however, necessary to make the transition from the point (a) to the point (b) immediately after the pressure decreasing operation, by the pressure increase compensating control so as not to affect the anti-skid control.
Therefore, in the case where the steering control by braking and the anti-skid control are performed simultaneously with respect to a wheel to be controlled, e.g., where the steering control by braking is performed when the wheel is under the anti-skid control, or where the anti-skid control is performed when the wheel is under the steering control by braking, if the cornering force is decreased rapidly, the steering control can be performed without affecting the anti-skid control. Furthermore, with respect to a desired slip rate set for the steering control by braking, it might be excessive for the anti-skid control. Therefore, when the anti-skid control is performed with respect to the wheel to be controlled, it is necessary to provide a certain limit for the desired slip rate.